Glow discharge tube



Feb. 8, 1955 PENNlNG ET AL 2,701,849

GLOW DISCHARGE TUBE Filed Feb. 14, 1947 HANS MICHELPEMVBVG,

TOMJUERLAAN/SE JOASEFAUGUASTMOUBLS.

INVENTORS,

AGIZNI August ;-Mubis, .Eindhoven, Netherlands, .assignors :to

Hartford National and .Trust JCompany, Hartf-f ord, Conm, as trustee.

Application February 14, 1947, Serial No. 728,530 In the Netherlands April, 1944' Section 1,'Public Law'690, Augus't' 8,1946- Patent expires'April"22,19'64 4 :Claims. ((31.313-178) invention relates -to a glow discharge 'tubefor 1156938 an :active'zelement in an electric circuit arrangement, more .particularlyfor measuring. or stabilisation purposes, and to a method of .manufacturingit. These glow discharge. tubes differ fromzglowlamps .in' that in using=:them it -1is,not the radiated .light but the properties tot-their characteristic that are utilised. The invention hasforits purpose to provide a tube with. which the voltage remains constant in practice during its life and which is reproducible but for a few volts.

Tubes of this kind .have hitherto/been provided with a.-.cathode of .-iron ornickel, sometimes coveredwith. an alkalinexearthvmetal. For purifying the. 'gasfilling they often contain a-getter consisting of -'barium:or magnesium.

United States Patent Wehave now found that the properties. of'these Itubes can be greatly improved and in .addition.the method. of manufacture :can be simplified- Accordingto the invention--.the cathode of a glow discharge tube consists, at least .at its. surface, of a metal having a melting .point exceeding 1400*0. and the wall of the tube, .at least the =wal-lzsurround-ingthe dischargepath, is practically entirely coated-with a visible layer 'of one/of. these metals... According to the invention the cathode and thewall coating preferably consist of the same metal. Thecathode .may entirely bemade from anyof the said metals" It .is sufficient, however, that solely the surface iayer of the .cathode consists thereof; We obtained-excellentresults with .a molybdenum cathode. Beside the cathode the inner 'wall of thetube must be :coated .with alayer 'of.:a.metal .havinga melting point 'ofzmore than 14009 C... To'such end it :is .not sufncient'tocoat the innerwall :with an invisible metal layer having a; thickness ef'a few .atomsonly. The layer;:must berclearly visible and practically entirely cover the wall surrounding the discharge path. If the discharge path within the glass-.renvelope Lot-the discharge tube be surrounded by a screen the lat-te r.will.ser=ve as-a wall for the discharge path and will "have to be coated with one of the aforesaidrmetals. Theexpres'sion visible Inetal'layer is to be understood to mean alayer which, when proyide don-taglass wall, absorbs about 20% of transmitted light. In general, the results will be better as -a--.greater part of the wall is covered and'for this reason the envelope of the discharge tube will be chosen in such manner that there are no dead spaces, such as tube arms, and the wall of the tube is substantially equidistant from the cathode surface throughout.

We have found that a glow discharge tube in which, according to the invention, the cathode surface and preferably also the Wall coating consists of molybdenum, has particularly suitable properties in regard to a constant operating voltage and reproducibility.

The glow discharge tubes set out above may, according to the invention, be realized by subjecting the cathode in the tube itself to a purification process by means of disintegration or vaporization and to provide the wall coating likewise by disintegration or vaporization. For instance, the cathode may be purified by high frequency heating for a long time to a high temperature. Preferably, however, the cathode is disintegrated by means of a glow discharge with a current density corresponding at least to several times the normal current density. To save time the cathode may with advantage also be purified with such density of the glow current that the cathode is heated to incandescence. The purificaice 'tion process should takeplace during manufacture offthe quired for thissealing operation, impurities from "the glass may occur. I It h'asturnedputthat 'after su ch a treatment detrimental mpn ti s a n ee i e .ofi duringnn- "era'tion ofzthe tube.

The wall coating of the'glow discharge itubemayjb'e a ze by h a e f t e a ores i meta s h the form of a wire or plate'in such manner, or bydisintegrati the .m j by e n of a glow disc ar n su mans that it P tical enti co e s. h -Wa l.wh ch su rounds the discharge path, with a'vi sible metalflayer.

The method according to the invention v" ldYIhe greatly simplified when usingjfor the wall. coating the same metal as for the cathode. In this waypurification-of the cathode and the coating of the .wall canbe realized simultaneously by cathode disintegration, which has to "be continued 'as long as is necessary forprecipitating an adequate wall coating.

'We .have foundthat the'operatingvoltagexof glow dis.- charge tubes according, tothe invention is extremely constantwithin wide time limitsand may, for instance, vary less than 0.5 volt in a normal life of T000 b urningghours.

The metals used, according .to the invention,-to constitute the material'for the cathode surface andathewa'll coating have-a melting point exceeding .l400""C. When the price of such material, .forv instance Ti or Hf, would make the discharge tube too expensive, a choice will, in general, ,be made out'of'the metals Zr, Ta, Cr, Mo, W, Fe and {Ni. Among the last-mentioned metals .molyb denum excels in that it has a low cathbde=anodevolt age in the glow current range volts in argon or neon) and by the facility with which the cathode of th is'metal can be "prepared in such manner that the glow covers the cathodefhomogeneously. In addition molybdenumhas the well-known properties of easy workability and high loading capacity. Asa result of the w cathode-anode voltage in the glow current range .the discharge does not settle at the poles, which often'consist'of metal, so that the poles need'not be shielded. In order "that the invention may be iqleallyfunderstood and readily, carried'into. effect it will now "bedescrihed more fully by giving several executiona'l examples which are illustrated 'in the accompanying drawing in' which: Fig. 1 is a sectional elevational view through the longitudinaliaxis of a glow discharge tube embodyingthe in.- ventlon.

Fig. 2-is a' hor'izontal sectional view of the cathode of the tube of Fig. :1. Y x

Figs. 3, 4 'and'S are sectional elevational viewsfof other embodiments of the tube of invention.

e eee h s "1 n .ms ate n de Tare contained-in an envelope? whichisfurnishedwvithkxe tensions 4'and'5'. The cathodelis'developeddnFig; 2. The inner wall of the envelope is covered with a dark metal deposit which, similarly to the cathode, consists of any of the aforesaid metals and preferably of the same metal. The inner wall of the extensions 4 and 5 is practically not covered with metal deposit, since the vaporized or disintegrated metal cannot reach the interior of these extensions. In the form of construction shown in Fig. 3 this drawback is met, since in this case the exten- SlOIlS are missing and the envelope is substantially equidistant from the cathode surface throughout. In this way the inner wall may be very evenly covered with metal deposit.

Fig. 4 represents a tube having a cylindrical cathode 1, an anode 2 and an envelope 3 which, similarly to the tube shown in Fig. 3, is substantially equidistant from the cathode surface throughout.

Fig. 5 represents a glow discharge tube having an envelope 3 in which the space of the discharge path between the cathode 1 and anode 2 is surrounded by a cylindrical quartz Wall 6. This wall is internally coated with a visible layer of metal 7, whereas the tube envelope shield ed by the cylinder is left clear.

For the manufacture of a glow discharge tube according to the invention use may be made of a tube as shown in any of the figures. In describing the method it is supposed that both the cathode and the wall coating consist of molybdenum and the cathode is purified and the inner wall is coated both by means of disintegration. First of all, all glass parts of the tube are degasified in a well-known manner by heating in a furnace at about 400; the metal parts are also degasified as thoroughly as possible, for instance by high-frequency heating at a high temperature. After that the tube is filled with gas and now a glow discharge can be struck between the cathode and the anode by means of a small current. When slowly raising this current it is observed that the cathode glow contracts to a small spot, the operation voltage falling off. After that the glow slowly spreads again over the surface of the cathode. Finally the current is raised to about several hundreds of milliamperes per cm. so that the cathode may be heated to white incandescence. Now the gas may be pumped away and the tube filled with a new quantity of pure gas. After sealing off the tube the disintegration process is repeated once more, so that ultimately both the cathode and the gasfilling are thoroughly purified so that practically the last traces of impurities are removed. At the same time the inner wall of the bulb is coated with a visible layer of molybdenum by disintegration of the cathode. In general the cathode will have been purified before the metal deposit on the inner wall has acquired a suflicient thickness. Consequently, it will be necessary to continue the disintegration until the inner wall of the tube is practically entirely covered with a visible layer of molybdenum.

Within the scope of the invention many variations of the method set out hereinbefore are possible. For instance, the molybdenum may be replaced by any of the other metals referred to both in regard to the cathode and for covering the wall. When using, for instance, a cathode of zirconium and a wall coating of nickel, a nickel wire or plate may be provided in the tube in the proximity of the cathode, so that upon disintegration nickel deposits on the inner wall. The inner wall coating may also be realized by heating the nickel wire by means of an electric current to such a high temperature as to precipitate nickel on the inner wall by vaporization of the wire. As an alternative the cathode may consist of a wire which is purified in the required manner by heating it to a high temperature by means of an electric current. For purifying the cathode it is not necessary to perform the disintegration in such manner that the cathode is heated to an extremely high temperature (in the case of molybdenum, for instance, to 1700"). In fact, this disintegration may take place with a smaller load and consequently at a lower temperature when making it last longer.

The gas pressure in glow discharge tubes according to the invention has the usual value of the order of magnitude of 1 cm. Thus, for instance, a gas pressure of 20 to 40 mm. neon or 5 to 20 mm. argon may be used. Glow discharge tubes according to the invention have a very constant operating yoltage. If the disintegration or vaporization of the cathode is continued for a sufiicient time and a visible layer of metal is precipitated on the inner wall of the tube, the operating voltage may vary less than 0.5 volt and even only 0.1 volt within a normal life of about 1000 burning hours. The operating voltage is bpractically not affected by interruptions in using the tu e.

What we claim is:

1. A glow discharge tube comprising a gaseous medium for producing a gaseous discharge, a cathode for said gaseous discharge having surface portions consisting of molybdenum, an anode for said gaseous discharge and defining with said cathode a gaseous discharge space, a wall portion enclosing said discharge space, and a molybdenum coating of visible thickness substantially entirely covering the surface of said wall portion facing said discharge space.

2. A glow discharge device comprising an envelope, a gaseous medium within said envelope, an anode electrode and a cold cathode electrode enclosed within said envelope to provide a glow discharge therebetween, a substantial portion of the interior surface of said envelope being exposed to said glow discharge, a metal coating on said cathode for determining its work function, said coating having inherent gettering properties, and a coating of the same metal on substantially the entire inside surface of said envelope exposed to said discharge to inhibit the liberation of gas from said surface, said latter coating being electrically isolated from said electrodes.

3. A glow discharge device comprising an envelope, a gaseous medium within said envelope, a rod-shaped anode electrode mounted within said envelope, a flat cold cathode electrode within said envelope, and spaced from said anode electrode to provide a glow discharge therebetween, a metal deposited on a portion of the surface of said cathode electrode for determining the work function of said cathode electrode, said metal having inherent gettering properties, and a metallic coating on the entire inside surface of said envelope exposed to said discharge to inhibit the liberation of gas from said surface, said metallic coating including the same metal deposited on said cathode portion, said latter coating being electrically isolated from said electrodes.

4. A glow discharge device comprising an envelope, a gaseous medium within said envelope, an anode electrode and a cold cathode electrode enclosed within said envelope to provide a glow discharge therebetween, a substantial portion of the interior surface of said envelope being exposed to said glow discharge, a metal coating on said cathode for determining its work function, said metal coating having a melting point greater than about 1400 C., said coating also having inherent gettering properties, and a coating of the same metal on substantially the entire inside surface of said envelope exposed to said discharge to inhibit the liberation of gas from said surface, said latter coating being electrically isolated from said electrodes.

References Cited in the file of this patent UNITED STATES PATENTS 1,840,130 Rashevsky Jan. 5, 1932 2,008,545 Dobke et al. July 16, 1935 2,167,777 Rentschler et al. Aug. 1, 1939 2,556,254 Carne June 12. 1951 

1. A GLOW DISCHARGE TUBE COMPRISING A GASEOUS MEDIUM FOR PRODUCING A GASEOUS DISCHARGE, A CATHODE FOR SAID GASEOUS DISCHARGE HAVING SURFACE PORTIONS CONSISTING OF MOLYBDENUM, AN ANODE FOR SAID GASEOUS DISCHARGE AND DEFINING WITH SAID CATHODE A GASEOUS DISCHARGE SPACE, A WALL PORTION ENCLOSING SAID DISCHARGE SPACE, AND A MOLYBDENUM COATING OF VISIBLE THICKNESS SUBSTANTIALLY ENTIRELY COVERING THE SURFACE OF SAID WALL PORTION FACING SAID DISCHARGE SPACE. 